All multicellular organisms comprise material with tensile strength and rigidity, such as bone and collagen, to maintain shape and to facilitate mechanical movement. Additionally, however, such organisms also must comprise a component with intrinsic elasticity, a component that can stretch and then undergo elastic recoil when required. For warm-blooded animals, this elasticity component is an unusually fibrous protein, elastin. Although elastin is present in virtually all tissue in some animals, it comprises an appreciable percentage of all protein in only some tissues, such as the arteries, some ligaments and the lungs. The elastin content of the human lung is about 28%.
Elastin can be hydrolized or otherwise destroyed by a select group of enzymes classified as elastases. The elastases are derived from many tissues in man, including the pancreas, neutrophils, macrophages, monocytes, platelets, smooth muscle cells and firbroblasts. Although called elastase, these enzymes are not just elastin-specific, and have been shown to cleave other proteins.
The role of elastases in normal elastin metabolism is difficult to assess, but a role in protein turnover is assumed. Human neutrophil granulocytes are the source of neutral proteases, human leukocyte elastase (HLE) and human neutrophil elastase (HNE), capable of hydrolysing most connective tissue components. However, the most likely primary physiologic substitute is elastin.